1. Field of the Invention
The invention relates to an apparatus and method for video transcoding, and more particularly, to a transcoder and related method operating in a transform domain for video coding schemes possessing different transform kernels.
2. Description of the Prior Art
The development of multimedia systems has necessitated that people devote attention to the problem of interoperability between different systems and different networks. To achieve a goal of interoperability among the various different systems and networks, devices such as gateways, multipoint control units, and servers are developed to provide a seamless transmission or seamless access of a multimedia content from content creation to content consumption. Transcoding of video content is one key technology that can make this seamless environment possible. Transcoding of video is defined as the operation of converting a video from one format into another format. A format is defined by characteristics including but not limited to: bit-rate, frame-rate, spatial resolution, coding syntax, and content.
A video transcoding can always be accomplished in a pixel domain. This is achieved by firstly fully decoding a video signal. The video signal was originally encoded in a first format. Because of fully decoding the video signal, a pixel-based content type signal is produced. Next, following the decoding, an encoding scheme is applied to re-encode the pixel-based content type signal into a video signal of a second format. This approach, however, requires complicated transcoding architecture and is generally not applicable to an apparatus having less computing power. Alternatively, for a block-based video coding scheme using discrete cosine transform (DCT) and motion compensation (MC), a transform-domain transcoding is preferred. In the transform-domain transcoding, a digital video signal of a first format is converted into a second format in DCT domain, allowing one to reduce the system complexity as well as maintain acceptable video quality.
H.264/AVC, a latest international video coding standard jointly developed by the ITU-T Video Coding Experts Group and the ISO/IEC Moving Picture Experts Group (MPEG), achieves higher coding efficiency by employing techniques such as variable block-size motion estimation and mode decision, intra prediction, and multiple reference frames. The emergence of H.264 introduces a non-trivial transcoding problem between itself and the MPEG series standards. This problem does not occur when transcoding video signals between the MPEG series standards themselves. However, H.264 adopts a modified discrete cosine transform (MDCT). The MDCT possesses a transform kernel different for that of the MPEG series standards. Additionally, the H.264 standard utilizes many new techniques that are not defined in MPEG-2 standard. In short, to transcode a digital video signal from an MPEG series standard (e.g., MPEG-2) into H.264 standard, one must overcome issues including but not limited to: transform kernel conversion, motion vector determination, macro-block mode selection, transform block size conversion, and transform-domain intra prediction.